Japanese patent publication 10-299846A discloses a conventional tension adjusting device for an engine accessory driving belt. Like the belt tension adjusting device according to the present invention, which is shown in FIG. 1, this conventional tension adjusting device includes a pulley arm 2 pivotally mounted on an engine block, a tension pulley 1 rotatably mounted on the pulley arm 2 so as to be in contact with the slack side of the belt B, and a hydraulic auto-tensioner 20 for applying a regulating force to the pulley arm 2, thereby pressing the tension pulley 1 against the belt B. The hydraulic auto-tensioner 20 includes a built-in hydraulic damper for damping a pressing force applied to the pulley arm 2 from the belt B through the tension pulley 1.
In the conventional arrangement, as shown in FIG. 5, the pulley arm 2 has a boss 3 at its end in which a shaft inserting hole 60 is formed to extend along the central axis of the boss 3. A pair of tubular slide bearings 61 each having a flange 61a at one end thereof are pressed into the shaft insertion hole 60. A fulcrum shaft 62 is further inserted in the slide bearings 61. The fulcrum shaft 62 has a small-diameter threaded shaft portion 63 at its front end which is threaded into a threaded hole 65 formed in the engine block 64 and tightened to support the pulley arm 2.
Seal grooves 66 are formed at both ends of the fulcrum shaft 62. O-rings 67 are fitted in the respective seal grooves 66 and each elastically in contact with the inner periphery of one of the pair of slide bearings 61 to prevent entry of muddy water and dust into between the contact surfaces of the slide bearings 61 and the fulcrum shaft 62.
Also, in the conventional arrangement, as shown in FIG. 6, the hydraulic auto-tensioner 20 has a coupling piece 68 formed with a bushing insertion hole 69 extending therethrough from one to the other side thereof. A tubular slide bearing 70 is pressed into the bushing insertion hole 69. A bushing 71 is inserted in the slide bearing 70. A bolt 73 is inserted through the bushing 71 and a pair of washers 72 each in abutment with one of the end surfaces of the bushing 71, and is threaded into a threaded hole 74 formed in the pulley arm 2 and tightened. The pulley arm 2 is thus coupled to the hydraulic auto-tensioner 20.
A pair of seal grooves 75 are formed in the outer periphery of the bushing 71 each near one end thereof. O-rings 76 are each received in one of the seal grooves 75 and elastically in contact with the inner periphery of the slide bearing 70 to prevent entry of muddy water and dust into between the contact surfaces of the slide bearing 70 and the bushing 71.
In the supporting structure of the pulley arm 2 of the above-described conventional belt tension adjusting device, and the coupling structure for coupling the pulley arm 2 to the hydraulic auto-tensioner 20, when the belt B vibrates at high speed while the engine is running, the pulley arm 2 pivots about the fulcrum shaft 62 at high speed, and the pulley arm 2 and the hydraulic auto-tensioner 20 also pivot relative to each other about their coupling portions at high speed. Thus, by contact with the slide bearings 61 and 70, the O-rings 67 and 76 tend to become worn and suffer deterioration in sealability, thereby making long use of the entire device difficult.
Also, in order to mount the O-rings 67 and 76, the seal grooves 66 and 75 have to be formed. In order to optimally control the interference of the O-rings 67 and 76, such seal grooves 66 and 75 have to be finished with precision, so that the manufacturing cost tends to be high.